This invention relates to optical data storage systems in general and in particular to optical data storage systems which can both record and read optically discernible data from a storage disk. Still more particularly, this invention relates to optical data storage systems in which the data can be selectively erased.
Optical data storage systems are well known in the art. The so-called "video disk" or "laser disk" systems are excellent examples of the impact of this technology in the marketplace. The optical systems utilized in these disks operate by focusing a laser on a rapidly rotating disk and reflecting light from the surface of the disk. The position of the laser may be accurately controlled utilizing a complex servo tracking system and the amount and characteristics of the light reflected from selected portions of the disk can be utilized to encode a large amount of digital data. The disks utilized with such systems are quite low in manufacturing cost and typically are constructed by ablating, stamping or burning small pits into the surface of each disk. By utilizing a laser beam to detect the presence or absence of these pits there is no mechanical connection between a stylus and the disk and therefore there is no stylus or disk wear. Additionally, a particular section of the data encoded on a disk may be accessed in a relatively short period of time and, in the case of an encoded television picture, a particular frame may be repeatedly accessed to provide a "freeze-frame" effect without adversely affecting either the disk or the stylus.
One shortcoming associated with optical disk data storage systems known in the art is that the typical system available in the marketplace can only be utilized to read data which has been imprinted on a disk at a manufacturing facility. Recently, newer systems have been proposed which can both read and write data onto a disk. Such systems typically use a laser at a slightly higher power level to ablate, burn or blister the disk surface and thereby provide a variation in the amount of light which can be reflected from the disk. While this system represents an advance in the art, it suffers from a shortfall in that the information recorded on the disk in this manner is permanently recorded and may not be erased.
In an effort to overcome this deficiency of known optical disk systems, several approaches have been proposed. In a first approach, a disk covered with a layer of certain rare earth materials is subjected to a magnetic field and then magnetized at selected points by heating those points to a point above the Curie temperature of the material by means of a laser. The presence or absence of magnetization in particular areas of the disk can then be detected by reflective reading with a polarized light source and by detecting the slight rotation of light caused by the Faraday effect.
A second approach which has been proposed utilizes materials which can be changed from an amorphous to a polycrystalline state by the utilization of a laser. The light from a different wavelength laser reflected from a particular portion of each disk can then be utilized to determine whether that portion of the disk material is in its crystalline or amorphous state.
Both the aforementioned systems suffer from severe deficiencies in actual applications. The magneto-optic approach requires the utilization of polarized light and is limited to the larger geometries typically associated with magnetic materials (thus limiting its density of data storage) and the second approach necessitates the utilization of two different wavelength lasers.
Recently, systems have been proposed for utilizing thermochromic phase change materials to store optically discernible digital data. Although this approach demonstrates certain advantages over the previously enumerated systems, there have been numerous difficulties encountered in attempting to maintain the accurate temperatures required. Examples of this early work may be seen in U.S. Pat. Nos. 3,789,420 and 3,789,421.
Therefore, it should be apparent that there has existed a need for an optical disk data storage system which can both read and write data and which may be selectively erased.